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Based upon bedrock geology, glaciation, soils, climate, and plant and animal distribution, Illinois can be divided into 14 natural regions and 33 distinct sections (Figure 2). From north to south, Illinois is more than 400 miles long. Throughout this 400 miles there is considerable variation in climate, topography, and soils. The southern tip of Illinois contains cypress swamps like those of southern states. Bogs and other wetlands similar to those in Minnesota and Michigan are present in northeastern Illinois. Different types of prairie and forest remnants are present throughout much of the state.

Prairies were known from all but 9 of the 102 counties in Illinois, although the prairies were small in the southern part of the state compared to those of the north. Learn as much as you can about the prairies present in your part of the state, including their soils and plant and animals species composition. This information should serve as a basic guide to the establishment of prairie.

The landscape of Illinois once consisted of approximately 22 million acres of prairie and 14 million acres of forest (Figure 3). Prairies were largely restricted to the northern two-thirds of the state while forests were predominant along major streams and in the hill country of southern Illinois. In central Illinois, prairies were so prevalent that scattered wooded sites, known as "prairie groves," were the only sources of wood.

Included in the 22 million acres of prairie were mesic, black soil prairies, sand prairies, and dolomite prairies. Of these, the black soil prairies were the most abundant, occurring in north-central, central, and south-central Illinois. Sand prairies were present along the Mississippi, Illinois, Green, and Kankakee rivers and along Lake Michigan while dolomite prairies were present in northern Illinois. Loess and glacial drift hill prairies were present along the south-facing bluffs of the major rivers, especially the Mississippi and the Illinois River systems.

The prairies of the Driftless Section in northwestern Illinois contain plant species of the northern great plains, such as pasque flower, plains buttercup, and June grass. The sand prairies of the Illinois River and Mississippi River Sand Areas contain plants typical of the western plains, including prickly pear cactus, hairy grama grass, sand love grass (Eragrostis trichodes), silvery bladderpod, and Pattersonís bindweed. Two plant species are largely restricted to the Western Forest-Prairie Division; the prairie trout lily (Erythronium mesochoreum) in the Carlinville Section and bunch flower (Melanthium virginianum) in the Galesburg Sections. Stickleaf (Mentezelia oligiosperma), a distinctive western plant, is present in the hill prairies of the Middle Mississippi Border Division.

Although some of the primary plants, such as big bluestem and Indian grass, were present in many of the prairie of the various natural divisions, their species composition was different. Use the natural divisionsí concept in establishing prairie communities in your part of the state.


Geologic evidence indicates that four glacial advances have occurred on what is now the state of Illinois. In order of occurrence, they were the Nebraskan, Kansan, Illinoian, and Wisconsinan glacial epochs. The two most recent glacial advances, the Illinoian and Wisconsinan, are largely responsible for the uniform flatness that now characterizes much of the state. At the end of the Illinoian glaciation, only extreme northwestern and southern Illinois plus Calhoun County and portions of Pike, Jersey, Monroe, and Randolph Counties were left unglaciated (Figure 4). The glacier advanced to a site near the present City of Carbondale, the southernmost point of glaciation in the northern hemisphere. The southern half of the state that was covered by the Illinoian glaciation is known as the southern till plain.

About 15,000 years ago the ice from the Wisconsinan glaciation covered most of the northern and east-central parts of the state. It is this glaciation that was responsible for the system of moraines in the east central and northeastern Illinois (Figure 4). The area occupied by the Wisconsinan ice sheet corresponds to what would later become the Grand Prairie. This was the first major expanse of grassland encountered by the settlers after leaving the heavily forested areas of the eastern states.

About 12,000 years ago, the climate became warmer and the glaciers began to melt and retreat, forming very large glacial lakes. Several of these were contained by moraines near the present site of the City of Kankakee. As the glaciers continued to melt, the water eventually cut through the moraines and cascaded down what is now the Illinois River Valley, resulting in a huge flood known today as the Kankakee Torrent (Willman and Frye 1970).

The waters of the Kankakee Torrent carried tremendous volumes of sand and gravel downstream to the "Big Bend" at Hennepin where the river channel is narrow and entrenched in bedrock. Below Hennepin, where the river valley widens, the water lost its velocity and the sand and gravel was deposited. Other major samd deposits in Illinois may be found along the Mississippi, Kankakee, and Green Rivers, and along the shores of Lake Michigan (Figure 5). As these sand deposits dried, they ere exposed to wind action, resulting in large sand dunes. On these sands deposits a truly unique ecosystem developed in Illinois, the sand prairies.


Over 600 soil types are known from Illinois, and most of these have developed from windblown silt which overlies glacial till. The windblown silt of loess was deposited during times of glacial retreat. The most recent soils are associated with the area of Wisconsinan glaciation. The soils of southern Illinois, on the area of Illinoian glaciation, are considerably older.

Mollisols, the dark-colored soils that developed under prairie vegetation, occur mostly in the northern half or two-thirds of Illinois. These thick, dark soils were formed by the decomposition of vegetation that consisted mostly of prairie grasses and wildflowers. To be classified as a mollisol, soils must have a dark surface layer at least 10 inches thick, and an organic content greater than one percent. Mollisols occupy approximately 49 percent of the state (Fehrenbacher 1967).

The other major soil group in Illinois, the alfisols, are the light-colored soils that developed under forest vegetation. The largest contiguous area of alfisols is the Southern Till Plain in the southern third of the state. These soils are less fertile than the mollisols, and large areas of Southern Till Plain was occupied by slow-growing post oak flatwoods. Approximately 46 percent of the state is occupied by alfisols (Fehrenbacher 1967).

Another soil group in Illinois, the histosols, are soils that are high in organic content. These are wetland soils that generally have an organic content greater than 14 percent. Histosols are probably most common in the morainal district of northeastern Illinois, but they are present in seeps and marshes along rivers as well as the prairie potholes of the Grand Prairie.


After the glaciers were gone, the climate cooled and a boreal forest, similar to that found in northern climates today, covered much of the state. Nevertheless, the climate gradually became warmer and drier and the vegetation changed. Oaks and hickories replaced the pines and spruces. At this time, 12,000 to 15,000 years ago, the prairie began to make significant eastward expansions (Axelrod 1985). About 8,300 years ago, during a prolonged hot, dry era known as the xerothermic period, the tallgrass prairie became a major vegetation type in Illinois (King 1093).

The present climate of the grassland region of central North America is the continental type, characterized by hot, dry summers and cold, usually dry winters. Precipitation varies from nearly 40 inches in the eastern tallgrass prairie to as little as 10 inches in the short grass prairies of the western plains states. Within Illinois, the annual precipitation averages about 38 inches, but the southern part of the state receives about 46 inches and the north receives about 34 inches annually, mostly as rainfall during the spring and summer months (Neely and Heister 1987).

The climate of Illinois is influenced by three air masses. The coldest and driest air originates from Canada and is most frequent in winter. Warm, very humid air originates from the Gulf of Mexico during the summer and dry, warm air from the Pacific Ocean influences our weather pattern, especially in the fall during what we refer to as Indian Summer.

Illinois is subject to considerable climatic variability, including periodic and frequently severe droughts. Reconstruction of past climatic conditions in Iowa, Illinois, and Missouri using tree ring analysis indicates very severe droughts in the 1890ís, and the dust bowl years of the 1930ís (Blasing and Duvick 1984). The five driest decades, in the last 300 years, were: (1) 1816-1825, (2) 1735-1744, (3) 1696-1705, (4) 1931-1940, (5) 1791-1800. This research indicates that severe droughts, lasting for a decade, were a relatively common phenomenon on the prairies. Severe droughts similar to the dust bowl years of the 1930ís can be expected to occur at least twice during every century (Duvick and Blasing 1981). Henry Allan Gleason, a renowned plant ecologist and geographer from Illinois, stated that the environmental extremes (floods, severe droughts, cold winters, late spring freezes) were climatic factors that had the greatest influence upon the distribution of plants.

The prairie ecosystem was modified and shaped by climate, fire, soils, topography, geology, glacial history, grazing pressures and time. Within the prairie biome, there are many different prairie types, each having its own distinct plants and animals. Corresponding to a steady decline in precipitation from east to west toward the Rocky Mountains, the prairies changed from tallgrass, to mixed grass, to the short grass prairie of the western plains.


The prairies of central North America are lands characterized by a nearly level to gently rolling topography. This is the type of terrain which provides a barrier free surface for the movement of fire. Fires eliminate the accumulation of dead leaves and stems of prairie plants and retard the encroachment of trees and shrubs. Trees and shrubs have vulnerable living tissue above ground and, therefore, are subject to the intense heat of a fire. In contrast, most prairie plants are deep rooted perennials that go dormant in the autumn and winter months leaving only dead, extremely flammable tops exposed by to fire.

While climate had a major influence on determining the distribution of prairies, geographer Carl Sauer, in writing about grasslands, stated that climate alone was not sufficient to explain the presence of the extensive prairies and savannas of the world.

In Illinois, fire was used by Native Americans in hunting buffalo, deer, and other game. They used what was called a ring fire or surround in their hunting. Sometimes these fire hunts were organized by a fire chief or leader. The hunting party would set out before dawn to surround a herd of bison. When the sun had dried the grass to the point where it would burn, the fires were started and the hunt began. Penalties were severe if anyone caused the bison to stampede before the hunt (McClain and Elzinga 1994). Father Hennepin described the use of fire by the Miami Tribe near the present site of Kankakee in December of 1679:

Until their departure from Illinois in 1832, Native Americans continued to use fire in their hunting. As the pioneers arrived, some of them apparently adopted this practice because prairie fires continued to be fairly common. These fires imperiled prairie travelers and homesteaders who lived near the praries. The fires moved with tremendous speed, stopping only where major rivers provided a break in prairie vegetation. One such conflagration in Illinois is described below:

As more pioneers arrived, prairie fires were discouraged due to potential harm to livestock, building, crops and people. Roads were constructed and cultivation of the prairie began. These activities created effective firebreaks and diminished the spread of prairie fires, the force that maintained the prairies by killing trees and other woody growth. It wasnít long before large trees were standing where prairie once grew.

Fire was a friend of the prairies, but it was greatly feared and hated by the pioneers. Some called it the "Messenger of Death". Uncontrolled fires were not compatible with their life style, so they worked hard to prevent them by burning late in the spring, by plowing and backfiring strips around their settlements, using cool season grasses, and by overgrazing the land. Getting rid of the fires meant getting rid of the tall, extremely flammable native grasses, and that meant getting rid of the prairie.

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